The invention relates to a new method for concentrating and dehydrating sewage sludge including activated sludge of an aerated biologic clearing step.
The aim to be achieved by the methods in industrial and/or local bio-aerobic sewage clarification is the production of an easily filtrable, compact sewage sludge having a high amount of dry substance contents in the filter cake. The reduction of the volume of sludge accompanying a high amount of dry substance contents by separating off the water portion permits a cost-effective and energy-saving further treatment of the sewage sludge by storing it on dumping grounds, burning it or using it in agriculture.
Sewage sludge consists of the so-called primary sludge obtained in the preliminary sedimentation by precipitation in presence of organic and/or inorganic flocculators and of the activated sludge produced in the biologic clearing step. Depending on the type and operating mode of the sewage treatment plant the sewage sludge contains between about 10 % to 90 % by weight of activated sludge. While the primary sludge corresponding to its origin by precipitation in the preliminary sedimentation basin generally shows good filterability properties (dry substance contents and filtration velocity), mixtures out of primary sludge and activated sludge as a rule are difficult to concentrate and dehydrate.
According to the state of art a so-called flocculation aid is added to the activated sludge, the so-called excess sludge, originating from the biologic clarifications step in the secondary sedimentation basin for improving the sedimentation and dehydration behaviour. The thus pretreated excess sludge is mixed with the primary sludge in the secondary concentrator and the sludge mixture is dehydrated in centrifuges, band filter presses or chamber filter presses.
The adding of flocculation aids can also be effected in another step, e.g. in the so-called secondary concentration--dosed into the mixture of primary and excess sludge--instead of being effected in the secondary sedimentation basin. The filter or centrifuge cake of the sewage sludge always contains more or less large portions of organic and/or inorganic flocculation aids apart from primary and activated sludge. The composition and the way of use of flocculation aids is known to the expert. In a largely used method the sludge to be filtered is treated with iron(III)salt, e.g. iron(III)-chloride and calcium hydroxide, in such manner that about 20 to 60 kgs of iron(III)chloride and 75 to 180 kgs of calcium hydroxide per ton of dry solid substance are used. By this way of action a compact, easily filterable sewage sludge is obained which, however, is loaded with additional amounts of slag-forming inorganic substances.
According to this so-called iron-lime method preferably organic polymeric flocculators are used for concentration/dehydration of sewage sludge including activated sludge. Polymeric flocculators are water-soluble, chain-forming polymerizates mostly produced by polymerization of acrylamide (e.g. as defined in the German publication 2,025,725 or 2,337,337 or Japan Kokai 75/161,479 or Japan Kokai 75/53,274 or Japan Kokai No. 74/59,855). It is, however, also possible to condense other polymerizable monomers, e.g. ethylene oxide (USSR Patent No. 1,204,576), ethyleneimine, allyl guanidines (U.S. Pat. No. 3,878,170) or quarternary carboxylic acid esters (Japan Kokai No. 76/04,084) or polyamines (U.S. Pat. No. 3,174,928, Spanish Patent No. 287,939, Japan Kokai Tokio Koho No. 86/192,734) to polyelectrolytes in chain form. In dependance on their electrostatic charge in aqueous solution a distinction is made between nonionnic, anion-active and cation-active polymeric flocculators. The combination, too, of inorganic/organic agents, such as pulverized brown coal plus polymers, is known for use as flocculator to the expert.
Per ton of solid substance about 1 kilogram to 7 kilograms of polymeric polyelectrolyte are required for achieving a sufficiently fast concentration and dehydration of the sewage sludge. By the use of organic polyelectrolytes the problem of the large amount of slag-forming inorganic substances in the filter cake of the sewage sludge (iron-lime method) is solved, however, the organic flocculators used instead represent a substantial cost factor which can decisively reduce the economy of sludge concentration and dehydration.
Therefore, numerous attempts have been made to modify the operation mode of sludge concentration and dehydration by means of the above-named organic polymeric flocculators in such manner that the required amount used is reduced to a minimum.
Japanese patent specification Kokai Tokio Koho No. 79/113,954, e.g., teaches the use of cation-active non-ionic copolymers of definite molar conditions for increasing the efficiency in the dehydration of sludges. European patent application 159,178 as well as Japan Kokai Tokio Koho No. 87/102,892 teach the use of synergistically acting inorganic/organic flocculating/charging agents for the purpose of saving polymers. In the patent specification Japan Kokai No. 75/141,850 a method is described for concentrating/dehydrating sewage sludges with reduced polymer consumption, in which method the sludge primarily is treated with a part of the sedimented clarifying agent of a previous precipitation, whereupon concentration of the pretreated sludge is effected using a reduced amount of fresh polymer. Japan Kokai No. 75/86,476 describes a synergetically acting combination of aluminium salts and non-ionic polyacrylamide for sludge dehydration.
The above-described synergetic methods require an accurate survey of the properties of the sludge to be dehydrated as well as a continuous checking of the precipitation/dehydration process for being able to make use of the advantages cited in the patent specifications. Japan Kokai Koho No. 87/132,600 describes, e.g., the analytic expenditure for predicting the dehydration behaviour of the sewage sludge. With the large variations of the flocculation and dehydration behaviour of mixed sludge ocurring in local and industrial sewage sludge treatment the use of the above-mentioned synergetic methods is limited.
It is, therefore, the object of the present invention to find a method for concentrating/dehydrating sewage sludge, which method permits a real reduction of the required amount of organic and/or inorganic flocculation agents independently from variations in the flocculation and dehydration behaviour of sewage sludge comprising activated sludge.
It has been found that said problem can be solved in accordance with the present invention in that the activated sludge portion of the sewage sludge to be dehydrated is produced in an aerated biologic clarification step in the presence of at least one compound selected from the group consisting of folic acid, dihydrofolic acid and at least one salt thereof.